Hi @JudgeyK,

OneMesh is a unique Mesh technology developed by TP-Link. Unlike traditional mesh systems, OneMesh allows you to use a mix of TP-Link Routers, Range Extenders, and Powerline Adapters, offering greater flexibility and cost savings. 
You may find the product models that support OneMesh here: https://www.tp-link.com/onemesh/product-list/
WiFi EasyMesh networks use a common standard that allows devices from multiple vendors to be onboard, configured, and able to communicate with each other. This brings flexibility, enabling devices from different vendors to work together to increase capacity and effective throughput by forming a smart and adaptable network. Not to mention, these networks are easy to set up and maintain. All can be self-organized to provide consistent coverage anywhere in a home, office, or outdoor space.
You may click here to see which models support EasyMesh: https://www.tp-link.com/easymesh/product-list/

In your case, you can consider easymesh to connect router + RE815XE. But you need to ask whether Linksys router supports easymesh.

If you want to connect RE815XE to router via Ethernet cable, you can consider Access Point mode.

For Tether App: https://www.tp-link.com/support/faq/2852/

For Web Management Interface: https://www.tp-link.com/support/faq/1401/